1. Field of the Invention
This invention relates to the art of dispensers of viscous fluids such as heavy oils, grease, caulking compound, mastic, putty, paste, and other fluids of high viscosity. More particularly the invention concerns viscous fluid dispensers such as syringes, grease guns, caulking guns and cartridges, and similar devices each containing an axially movable piston or plunger for discharging a heavy fluid or semifluid from a nozzle at one end of the dispenser. The invention is especially directed at a dispensing device of the type mentioned wherein the device has a cylindrical barrel containing the fluid material to be discharged from the nozzle, which barrel is filled by hydraulically or pneumatically injecting the viscous fluid through the nozzle.
2. Description of the Prior Art
Viscous fluid dispensers of the type mentioned above heretofore known, which are filled with fluid via the nozzle, have had the objectionable characteristic that air is generally entrapped at the piston head or seal when the dispenser barrel is filled with fluid. This entrapped air causes the following difficulties:
A. The exact desired dose of fluid cannot be discharged or administered because some or all doses contain air entrapped in the dispenser during the filling procedure.
B. The presence of the entrapped air further complicates the accuracy of calculation of the dose, because the entrapped air is easier to compress than the viscous fluid. The precise movement of the piston head or seal axially in the barrel of the dispenser is often taken as a measure of the dosage being dispensed, but this calculation is rendered inaccurate because it does not take into account the quantity of air in the barrel entrapped between the piston head and fluid.
C. The air compressed by the piston during the last administered or discharged doses of fluid, expands after forward discharging movement of the piston stops. This expanding air causes undesirable drooling, dripping, or leakage of fluid from the nozzle of the dispenser.
It has been proposed in U.S. Pat. No. 3,291,128 issued to J.G. O'Neill, dated Dec. 13, 1966, to remedy this problem of air entrapment by providing a very flexible, soft piston head, moved along fine grooves or ridges extending the full length of the barrel. The soft material of the piston head enters and seal the grooves or passages between ridges. Entrapped air is released by twisting the head circumferentially of the barrel. This design has not proved wholly satisfactory in practice for two reasons. Firstly, due to the required softness of the piston head, parts of the piston head sealing the grooves or passages are pushed out of the grooves when pressure is applied to the head to discharge fluid by advancing the head in the barrel. This condition allows the fluid in the barrel to leak backward out throught the long grooves or passages. Secondly, when the head is rotationally twisted, air and fluid both flow rearwardly through the grooves or passages and the fluid undesirably leaks out of the barrel.